This application relates to shaped plastic lenses and to a method for their manufacture. More particularly, the application relates to plastic lenses having substantially no optical power, high impact-resistant and abrasion-resistant properties and to a method for forming such lenses wherein a high scratch-resistant convex surface of the lens is formed in situ during the method.
Curved light-polarizing laminates useful as lenses and comprising a layer of a molecularly-oriented light-polarizing material arranged between a pair of substrate sheets are well known. It is known to manufacture composite light-polarizing lenses, which include a layer of an optical quality polymeric material on each side of a shaped light-polarizing member. U.S. Pat. No. 3,940,304 describes a technique, which includes in situ polymerization of the optical quality polymeric material layers in a mold. An optical quality monomeric material is inserted into a shaping mold so as to cover both surfaces of the light-polarizing member and heat is applied to cause polymerization of the monomeric material to occur thereby resulting in the formation of a composite polymeric light-polarizing lens structure.
Further, it is known to make lenses, which have substantially, no optical power by shaping a composite lens, blank in a mold. For example, U.S. Pat. No. 5,434,707 teaches a method for forming shaped plastic lenses having substantially no optical power and comprising a laminate of a light-polarizing layer arranged between a pair of thermoplastic substrate sheets. According to the method, the composite lens blank is inserted into a mold which has heated curved platens and subjected to heating and pressing such that the thermoplastic substrate sheets are deformed and rendered flowable with the result that there is produced a composite lens which has substantially no optical power. The method requires lens-forming platens having predetermined radii of curvature which make possible, under the heating and pressing conditions, the production of plastic lenses of non-uniform thickness, that is, lenses which are thickest in the central region and of diminishing thickness radially to the periphery thereof. Although the method taught by the ""707 patent provides shaped plastic lenses having substantially no optical power and of good durability, it is not completely satisfactory for all situations.
As the state of the art advances, efforts are made to provide new plastic lenses, which can meet new performance requirements, and to reduce or eliminate some of the undesirable characteristics of the prior art materials. For example, there is a continuing demand for high impact-resistant, plastic lenses, which have substantially no optical power for various uses such as in sporting events. However, simply making high impact-resistant plastic lenses by shaping in a mold a lens blank made of a high impact-resistant plastic material is not completely satisfactory since such high impact-resistant materials typically exhibit high optical stress properties and tend to develop cracks under the heating and pressure conditions required to shape the lens. Thus, it would be advantageous to have a method for making shaped lenses which have substantially no optical power and high impact strength and which are suitable for use in eyeglasses. It would also be advantageous to have shaped lenses which have substantially no optical power and which possess a high scratch-resistant convex surface.
It has now been found that shaped, or curved, plastic lenses having substantially no optical power can be provided according to the invention by forming, via in situ polymerization, a layer of an optically clear, high scratch-resistant polymeric material on the convex surface of the lens. The method of the invention is carried out in a mold, which comprises heated curved platens. An appropriate amount of a suitable polymerizable composition comprising a monomer or an oligomer is inserted onto the concave surface of one platen. A planar lens blank of substantially uniform thickness is interposed between the platens, which are then heated and pressed together. The polymerizable composition is thereby caused to cover the convex surface of the shaped lens structure and polymerize to form a layer of non-uniform thickness which is thickest in the central region and has a thickness gradient diminishing gradually radially towards the periphery of the lens.
In a preferred embodiment the lens blank which is utilized is a composite structure comprising a layer of light-polarizing material between a plurality of thermoplastic substrate layers. There is thus provided according to the invention lenses, which are suitable for use in sunglasses.
The advantageous shaped plastic lens thus formed, convex on one side and concave on the other side, has substantially no optical power and comprises one or more transparent layers of substantially uniform thickness on the concave side and, on the convex side, a transparent layer of thermoplastic material having high scratch resistance and non-uniform thickness, with the maximum thickness of this layer being in the central region of the lens and the thickness diminishing gradually radially toward the periphery of the lens. Thus, the advantageous lens structure of the invention has an overall non-uniform thickness with the maximum thickness of the lens being in the central region and the thickness diminishing gradually radially towards the periphery of the lens.
Further in accordance with the invention there is provided a method for forming a shaped plastic lens having substantially no optical power, convex on one side and concave on the other side, and having its maximum thickness in the central region and diminishing gradually in thickness radially towards its periphery, the method comprising the steps of:
placing a lens blank comprising one or more transparent substantially uniformly thick layers of thermoplastic material between opposed concave and convex platens for forming, respectively, convex and concave surfaces on the lens blank, the radius of curvature (r1) of the concave platen and the radius of curvature (r2) of the convex platen each corresponding substantially to the relationship             r      1        +          r      2        =      t    ⁡          (                        n          -          1                n            )      
xe2x80x83wherein t represents the thickness of the lens blank and n is the index of refraction;
placing a volume of a polymerizable composition on the concave platen; and
heating and pressing the platens together with the lens blank between them, the heating and pressing being such as to cause the lens blank to deform and to conform one surface to the convex platen and to cause the polymerizable composition to polymerize and form a transparent layer of non-uniform thickness conforming to the concave platen, with the layer thus formed in situ having its maximum thickness in the central region of the lens and diminishing gradually in thickness radially towards the periphery of the lens.
The advantageous method of the invention allows the manufacture of lenses which have high impact strength and a high scratch-resistant convex surface.